This invention relates to an aluminum support for a planographic printing plate, its production and roughening an aluminum support, suitable for offset printing, etc.
In general, aluminum plates are widely used as supports for a planographic printing plate. The surface of the aluminum plate is usually roughened for the purpose of the improvement in adhesiveness of a photosensitive layer provided thereon and the improvement in the water retention of nonimage area (the area which receives damping water used during printing and repels oily ink, and is carried by the area wherein the surface of the support is exposed) of the planographic printing plate produced using the same.
The roughening is called graining and requires a great deal of skill. The graining can be divided roughly into mechanical methods, such as ball graining, wire graining and brush graining, and electrochemical methods.
In the case of ball graining, there are many factors requiring skill, such as ball material, the type of abrasive and control of water amount during graining, and moreover, graining of plates must be conducted one by one because continuous graining is impossible. In the case of wire graining, grained surface is not uniform. On the other hand, brush graining brings uniformly grained surface, and continuous graining is possible. Accordingly, brush graining is suitable for mass production.
In any event, it is difficult to obtain a plate having sufficient performance for a support for a printing plate by the mechanical method mentioned above.
In general, it is said that the greater surface roughness brings the greater water retention, and in the case of producing a planographic printing plate, preferred supports have indentations as uniform as possible in order to improve water retention and printability. As a means for producing such a preferable surface, electrochemical roughening is noted. In the case of electrochemical roughening, aluminum plates having a uniformly roughened surface can be obtained by keeping various conditions, such as the composition and temperature of electrolytic solution, electrolytic conditions, etc.
The electrochemical roughening can be divided roughly into methods of using alternating current and methods of using direct current. The method of using alternating current has a disadvantage that unevenness tends to occur in the direction perpendicular to the advancing direction of an aluminum plate according to the frequency of the alternating current used for roughening and traveling speed of the aluminum plate.
A means for solving the above problem is disclosed in U.S. Pat No. 4,902,389 wherein anodes and cathodes are arranged alternately faced to an aluminum plate. DC voltage is applied between both electrodes, and an aluminum plate is passed with keeping a prescribed space.
However, according to the roughening using direct current only, scumming reduction is incompatible with fill-in reduction of ink at half-tone dot portions upon reducing damping water, and printability applicable to high grade printing cannot be achieved. In the roughening disclosed in U.S. Pat. No. 4,902,389 using direct current, roughening greatly depends on an apparatus, and in order to produce a surface shape suitable for the printability of the aluminum support for various planographic printing plates, electrolytic conditions must be greatly changed.